The place of metabolism in gerontology has thus far remained elusive. It is proposed here that senescence can be analyzed via a macroscopic entropy balance on a plant or animal. This balance clearly delineates and separates the effects of metabolism from those of nutrition, respiration and perspiration, and may provide information on life expectancy. Metabolic rate is directly related to the aging phenomenon in a macroscopic way through the entropy rate balance and the second law of thermodynamics. It is therefore conceivable that the aging process can be controlled via metabolic control. A similar speculation has been recently made by Rosenberg (10), et al., in predicting that small decreases in core body temperature of homeothermic animals should produce a marked increase in life expectancy. To test and develop this theory requires beginning with experiments on lower animals with relatively short life spans, say fishes, and to determine their metabolic rate via direct calorimetry. This can easily be done by putting the animals in a totally thermally insulated environment and thus creating essentially an isolated system. Then all metabolic energy generated must pass into the animal's local environment (and contained there due to its closed nature) and can be detected by accurately measuring temperatures and compositions therein. Obviously the local environment cannot remain permanently closed, but it can be closed long enough to periodically make such metabolic measurements.